The present invention relates generally to a heating element, and more particularly to a heating element disposed in a support plate for heating a substrate in a processing chamber.
A susceptor is a mechanical part that holds a substrate in a processing chamber for a fabrication step, such as chemical vapor deposition (CVD). The susceptor includes a substrate support plate mounted on a stem, along with a lift assembly for raising and lowering the substrate within the processing chamber. The substrate support plate is heated to facilitate the fabrication process. Typically, a heating element is disposed within the support plate. The heating element maybe a tube assembly which includes a heating filament surrounded by an outer sheath. The space between the heating filament and the outer sheath may be filled by a sealing material.
Heat density refers to the amount of heat which is generated by a heating element over a given length of the element. Power density is a related concept which refers to the amount of power (in watts) which is dissipated over a given length of the element. Heating elements having high heat and power densities are preferable in substrate fabrication processes to minimize the area of the support plate occupied by the heating element, while still enabling adequate heating of a substrate.
To achieve high heat and power density performance for the heating element, the sealing material should efficiently transfer heat from the filament to the outer sheath; that is, the sealing material should be thermally conductive. However, the sealing material should also be electrically insulative to prevent a short circuit between the heating filament and outer sheath. The outer sheath of the heating element may be constructed of a metal such incoloy, which is an alloy primarily composed of iron, nickel and chromium manufactured by The International Nickel Co., Inc. The sealing material may be magnesium oxide (MgO) or boron nitride (BN).
The susceptor plate undergoes heating and cooling each time a substrate is processed in the chamber. This repeated heating and cooling subjects the sealing material to considerable thermal stresses. The shock of repeated thermal stresses may reduce the thermal conductivity or increase the electrical conductivity of the sealing material. If the thermal conductivity of the sealing material decreases, heat will accumulate in the filament. As a result, the filament may burnout or the susceptor may be not adequately heated. If the electrical conductivity of the sealing material increases, then there may be a short circuit between the filament and the outer sheath. Thus, the sealing material should be resistant to thermal shock so that the heating element has a long lifetime.